The invention relates to a polyamide composition containing carbon nanotubes (CNTs) and/or graphene as electrically conductive carbon substrate and additionally a bifunctional or oligofunctional coupler.
Although there are a few specific exceptions, plastics are generally electrical insulators and when prepared as films or mouldings high surface charges can accumulate during preparation, processing and use.
These static charges may lead to undesirable effects and serious hazard situations which include attraction of dust, adhesion of hygienically problematical contamination, destruction of electronic components through arcing, physiologically unpleasant electric shocks, ignition of flammable liquids in containers or tubes used to stir, pour or convey the liquids, or dust explosions, which may occur, for example during transfer of dusts or fine powders into, between and from large containers or in rock quarrying or coal mining.
Therefore, when plastics are utilized in such manner, elimination or minimization of static charge accumulation is necessary to prevent such hazardous situations.
Conventional thermoplastics have specific surface resistances in the range from 1016 to 1014 ohm and can therefore build up voltages of up to 15,000 volts. Effective antistatics reduce the specific surface resistances of the plastics to from 1010 to 109 ohm. However, a significantly higher degree of removal of electric charges is required for plastics to be used in electronic components of large appliances, e.g. in the transformer or electric substation sector or in many applications in automobile and aircraft construction. where electrically conductive moulding compositions which have to have a specific surface resistance of less than 109 ohm are sought. A further critical aspect is that in such plastics applications not only the surface resistance but also the volume resistance through polymer parts having a thickness of up to a number of millimetres has to be in such a range and in the case of parts which are produced by injection moulding, anisotropic effects frequently occur and are generally difficult to prevent.
In conventional practice, a polymer composition containing carbon black and in particular conductive carbon black is used for the manufacture of conductive polymer parts. Conductive carbon blacks are fractal structures which are able to conduct the electric charge in the polymer by mutual contact and also guarantee a low volume resistance. High degrees of fill in the range from 15 to 25% by weight may be necessary for this purpose and such high content can have an adverse effect on the technical properties of the polymer, especially the mechanical properties such as notched impact toughness or tensile strength. Additionally, such high content may lead to an unsatisfactory surface quality for components which are to be visible, resulting in quality complaints.
Conductive carbon blacks are specific industrial carbon blacks which are produced, inter alia, by the furnace black process or by thermal dissociation in the acetylene black process. These conductive carbon blacks have a high specific surface area as indicated by DBP (dibutyl phthalate) values of more than 110 ml per 100 g of carbon black. Commercially available conductive carbon blacks include, for example, Ketjenblack EC, Printex XE2 and Printex L6.
In comparison, carbon nanotubes (CNTs) and especially graphene may be used in significantly lower concentrations than conductive carbon blacks. However, the dispersion of CNTs and graphene may be extremely process-dependent. For example, the method by which the CNTs are introduced into the extrusion screw at the feed end and the screw configuration selected in an extrusion process may be related to product effectiveness. These are only some of the relevant process parameters which must be considered for effective utilization. Furthermore, the costs of both CNTs and graphene are considerably greater than the price of conductive carbon blacks. Therefore, there is a need for further process and formulation optimization to lower the CNTs or graphene concentration and improve cost efficiency. In some plastics, dispersing these products is particularly difficult since the melt viscosity and the polarity of some thermoplastics are not conducive to dispersion.
Carbon nanotubes are a further modification of the element carbon in addition to graphite, diamond, amorphous carbon and fullerenes. In the nanotube structure the carbon atoms are arranged in hexagons. The structure corresponds to a rolled-up monoatomic or polyatomic layer of graphite so as to form a hollow cylinder typically having a diameter of a few nanometres and a length of up to a few millimetres. Carbon nanotubes may be distinguished as multiwalled and single-walled carbon nanotubes, usually abbreviated as MWNTs and SWNTs. Owing to van der Waals forces, carbon nanotubes have a strong tendency to agglomerate to form bundles, and as a consequence, detangling and dispersing without severe shortening by high shear forces may be essential in the extrusion process. Commercially available products may be obtained from various producers, such as, for example, Bayer, Nanocyl and Arkema and include various grades such as Baytubes® C150P, Baytubes® C150HP, Baytubes® C70P, Nanocyl™ NC7000 and Electrovac Graphistrength C 100. Further manufacturers offer CNTs in the form of masterbatches, for example Hyperion and C-Polymers.
Polyamide compositions containing CNTs are described in U.S. Pat. Nos. 4,663,230; 5,098,771; 5,578,543; 5,591,382; 5,611,964; 5,643,502; 5,651,922; 6,235,674 and 6,608,133.
However, agglomerates of incompletely dispersed fillers frequently occur in the compounding of polymers with conductive fillers, and these limit the product quality of the composition. For example, such agglomerates may lead to surface defects which cannot be tolerated in specific uses. In addition, the conductivity achieved at a given concentration of conductive filler falls far short of the theoretical achievable conductivity.
The assessment of the quality of dispersion may, for example, be conducted by optical microscopic examination of thin sections, with the proportion by area of the agglomerates being measured.
In the light of this background, it was an object of the invention to improve the dispersion of carbon nanotubes or graphene in polyamide moulding compositions. In one aspect of the object, the surface quality of the moulding composition should be improved in this way. In a further aspect of the object, the electrical conductivity should be improved at a given content of carbon nanotubes or graphene or lower filler content should be necessary for achieving a desired conductivity.